The present invention relates to a plasma processing apparatus. More particularly, the present invention relates to a method and apparatus for controlling the voltage applied to an electrostatic shield disposed between an induction coil and a plasma.
Inductively coupled type dry etching systems are commonly used in the semiconductor manufacturing industry. The dry etching apparatus generally has a process chamber with a ceiling of a dielectric wall, on which an annular or spiral radio frequency (RF) antenna is disposed.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil induces electromagnetic currents in a plasma. The currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. Typically, the current through a coil acts as the primary winding of a transformer. The plasma acts as a single turn secondary winding of the transformer.
During a plasma process, the RF antenna and a plasma excited in the process chamber are coupled not only inductively but also capacitively. Consequently, the inner surface of the dielectric wall made of, e.g., quartz, which is exposed to the interior atmosphere of the process chamber near the RF antenna, is charged with a negative bias relative to the plasma. With the potential difference between the plasma and the exposed inner surface of the dielectric wall, positive ions in the plasma collide with the exposed inner surface while being accelerated. As a result, problems arise in that contaminants are produced in the process chamber and the dielectric wall is worn off quickly.
In order to cope with these problems, a conductive Faraday shield is normally disposed between a dielectric window and an insulating layer under an RF antenna. The capacitive coupling between the RF antenna and the plasma is disrupted by the Faraday shield, so that the exposed inner surface of the dielectric wall is protected from collisions with accelerated positive ions from the plasma. The Faraday shield is preferably connected to a source of RF potential to control its relative potential or bias. One option is to connect the shield to a point from the antenna in which case the shield operates at the same frequency as the coil as illustrated in FIG. 1. However, the current that is coupled out of the coil reduces the magnetic coupling with the plasma. Although such a powering scheme is simple and efficient, the controls of recipe variables are therefore limited.
An alternative powering scheme uses an external second auxiliary RF power supply separate from the main supply that powers the coil as illustrated in FIG. 2. The advantages of this scheme are the option to operate the shield at a different frequency from the antenna, the substantially smaller interaction between the shield and the coil circuit, and simplicity of control. However such extra circuitry results in higher cost and may complicate control.
Accordingly, a need exists for an apparatus and method to independently control the antenna current and the Faraday shield voltage both powered by a single power supply.
An apparatus for controlling the voltage applied to a shield interposed between an induction coil powered by a power supply via a matching network, and the plasma it generates, comprises a shield, a first feedback circuit, and a second feedback circuit. The power supply powers the shield. The first feedback circuit is connected to the induction coil for controlling the power supply. The second feedback circuit is connected to the shield for controlling the voltage of the shield. Both first and second feedback circuits operate at different frequency ranges. The first feedback circuit further comprises a first controller and a first sensor. The first sensor sends a first signal representing the power supplied to the inductive coil to the first controller. The first controller adjusts the power supply such that the power supplied to the inductor coil is controlled by a first set point. The second feedback circuit further comprises a second sensor, a second controller, and a variable impedance network. The shield is powered via a variable impedance network. The second sensor sends a second signal representative of the voltage of the shield to the second controller. The second controller adjusts the variable impedance network such that the voltage of the shield is controlled by a second set point.